The present invention relates to a piston for an internal-combustion engine.
The piston for the internal-combustion engine is fitted in a cylinder with a predetermined clearance and reciprocates in the cylinder. At this time, because a connecting rod supports the piston in a state that the connecting rod inclines from rightward to leftward, the piston slides in a reciprocating motion in a state that the piston is pressed to the inner surface of the cylinder by a side force of right or left direction.
Because the side force is great and the effect of a partial thrust force on the inner surface of the cylinder causes a deficiency such as a seizure, it is effective for avoiding the deficiency that the thrust force is dispersed and the surface pressure is lowered by making the area the skirt section of the piston contacts with the inner surface of the cylinder become great.
However, if the area on which the skirt section of the piston contacts with the inner surface of the cylinder becomes greater, a friction loss during the sliding motion of the piston becomes greater.
Therefore, Japanese Unexamined (KOKAI) Utility Model Publication No. 64(1989)-3054 discloses an unsymmetrical piston in which the area of a thrust side skirt which contacts with the inner surface of the cylinder during an expansion stroke and is effected by a great thrust force and the area of a counter-thrust side skirt is small.
With this, it is possible to gain the piston for the internal-combustion engine in which the thrust force can be dispersed, the surface pressure can be lowered and the increase of the friction loss can be controlled.
When the piston reciprocates in the cylinder, if the side force changes the direction, it is apprehended that the piston moves from one side to the other side and the collision between the piston and the inner surface of the cylinder causes a piston slap and a noise.
As a countermeasure against the noise, it is effective that the lower portion of the skirt section of the piston loosely contacts with the inner surface of the cylinder. However, in the conventional example, the wall thickness of the skirt section is substantially constant. The dimension of the wall thickness is chosen so as to secure a rigidity bearable to a great side force at a time of a high output of an engine.
In the conventional example, the piston includes enough rigidity at the time of the high output of the engine and it is possible to control the friction loss during the sliding motion of the piston. However, it is apprehended that the noise is generated when the skirt section of the piston contacts with the inner surface of the cylinder.